The present invention relates to a machine for the production of plastic receptacles comprising a chassis and arranged along a guide path at least one heating device and a blow-molding device, the receptacles being formed from pre-forms disposed on conveyors moved in one direction of movement along the guide path, the heating device comprising heating elements mounted facing at least one row of pre-forms.
There are known such machines described for example in WO 97/13632. The machine described in this latter document has a heating device with several rows of parallel pre-forms. Heating tubes are disposed between two rows of pre-forms to heat these latter. When it is desired to change the tubes or to adjust their position, it is necessary to stop the heating device and to immobilize the machine for a prolonged period until a temperature suitable for handling the heating elements is reached. Then, the operations of maintenance and adjustment are difficult to carry out because the heating elements are disposed at the center of the machine and are therefore difficult of access. The known machine therefore requires prolonged down periods and complicated and impractical handling and adjustment operations.
U.S. Pat. No. 5,121,828 discloses a machine for the production of articles from pre-forms comprising a chassis with a base which has rails on which an oven for heating the pre-forms is mounted in a slidable manner. The assembly of the oven can thus be moved slidingly on the rails relative to its base. However, when a single heating tube is to be replaced or adjusted, it is necessary to withdraw and remove the assembly from the oven. Moreover, if this tube to be replaced is located in the middle of the oven, the access to this tube is far from being easy. It is necessary to wait until the temperature of the heating elements has reached a sufficiently low level to permit an operation of repair or adjustment. The down time for operations and maintenance and adjustment therefore becomes long even for this type of machine.
The present invention has for its object to overcome these drawbacks, and the machine according to the present invention is characterized to this effect by the fact that the heating device comprises, associated with the different rows of pre-forms, sliding mechanisms arranged so as to permit withdrawing and reintroducing individually the heating elements of a predetermined row relative to other heating elements associated with other rows of pre-forms and relative to a fixed frame of the heating device.
Thanks to these characteristics, it is possible to withdraw the heating elements of the heating device by simple sliding in the manner of a drawer, which assures easy access to the heating elements, and hence facilitated maintenance and adjustment of these heating elements. The cooling of these latter outside the heating device is moreover swifter, which significantly shortens the down time of the machine during operations of maintenance and/or adjustment.
According to a preferred embodiment, the machine comprises pairs of parallel rows of pre-forms at the level of the heating device and is characterized by the fact that the heating elements are disposed between the pairs of parallel rows and mounted on sliding mechanisms such that the heating elements associated with a pair of parallel rows can be withdrawn from the heating device relative to other heating elements associated with the other pairs of parallel rows by sliding in said direction of movement.
These characteristics permit easy removal of the heating elements for maintenance and adjustment.
Preferably, the sliding mechanism comprises two telescopic devices fixed above the two rows of pre-forms to the frame of the heating device, the heating elements being suspended from cross-pieces connecting the two telescopic devices.
There is obtained by these characteristics a particularly simple construction, a reliable operation and a reduced size.
Preferably, the heating elements are distributed in two heating bodies arranged one behind the other on the sliding mechanism separated by a predetermined distance.
The two heating separated heating bodies can thus be adjusted individually rapidly and easily, which permits an adaptation of the heating device to the production of receptacles and other hollow bodies, of different shapes and sizes and compositions.
In a preferred embodiment, each heating body comprises two parallel series of heating tubes substantially superposed and mounted facing two rows of pre-forms on vertical suspension bars connected to the sliding mechanism.
There is thus obtained a very efficacious and precise heating of the pre-forms, whilst having a construction greatly facilitating maintenance and adjustment of the heating elements.
The heating tubes are preferably mounted adjustably on suspension bars such that the distance separating the pre-forms can be individually adjusted.
These characteristics permit obtaining and adjusting heating profiles along the axis of the pre-forms assuring high quality of the produced products.
According to a preferred modification, the heating device comprises a frame with feet whose length can be adjusted by a simple mechanism with pinions and a chain such that the position of the assembly of heating elements relative to the pre-forms can be adjusted vertically.
An adaptation of the heating device to pre-forms of different lengths can thus be obtained in a rapid and precise manner.
Preferably, the heating device comprises at least three sliding mechanisms disposed parallel to each other and whose heating elements are disposed in the middle of three pairs of rows of pre-forms, each sliding 85 mechanism being arranged so as to permit withdrawing individually the heating elements located between two adjacent rows of pre-forms.
There is thus obtained a rapid and easy maintenance and adjustment, even in the environment of a heating device of large size having at least six parallel rows of pre-forms.
A preferred embodiment of the invention is characterized by the fact that the heating device comprises at least four successive zones arranged along the direction of movement, and ventilation means adapted to send an air flow along the surface of the pre-forms, a first zone comprising heating elements adapted to produce heating of the pre-forms, a second zone permitting balancing the thermal conditions of the wall of the pre-forms by cooling their external surface, a third zone comprising heating elements adapted to produce a second heating of the pre-forms and a fourth zone permitting a final balancing of the thermal conditions of the pre-forms before their transfer to the blow-molding device, the heating device having ventilation means in the heating zones and in the balancing zones, these ventilation means being arranged so as to send an air flow oriented perpendicularly to the surfaces of the pre-forms in the balancing zone and in the two heating zones, these latter having positions in which the pre-forms are simultaneously subjected to heating radiation and to said air flow.
Thanks to these characteristics, it is possible to obtain an optimum differential heating of the wall of the pre-forms, namely a given thermal profile through the wall of the pre-forms. There can thus be obtained an external surface at a lower temperature than the internal surface of the pre-form. The invention therefore permits obtaining an optimized thermal distribution for pre-forms ensuring a very regular expansion during the subsequent drawing-blowing operation.
Preferably, each of the four zones comprises a predetermined number of stop positions in which the pre-forms are stopped during predetermined time intervals, the ventilation means being arranged so as to direct in the stop positions a substantially vertical flow of air in the direction of the pre-forms, which are rotated about their longitudinal axes by means of at least one drive device.
The combined and successive heating-cooling actions and cooling alone, in the precisely determined stop positions of the pre-forms, ensure a particularly precise thermal distribution for the pre-forms, hence a very regular moving of the receptacles.
Preferably, the machine is characterized by the fact that the heating device is arranged so as to carry out in the first zone a heating and a ventilation such that the external surface of the pre-forms is at a temperature higher than that of the internal surface, to carry out in the second zone a balancing by ventilation such that the temperature of the external surface becomes lower than that of the internal surface, to carry out in the third zone a ventilation and general heating of the pre-form less pronounced than in the first zone, and to carry out in the fourth zone balancing conditions by ventilation such that the temperature of the external surface is lower than the internal temperature of the pre-forms at the outlet of the heating device.
Thanks to these characteristics, the temperature of the internal and external surfaces of the pre-forms can be adjusted in a precise manner whilst obtaining a predetermined temperature profile through the walls of the pre-forms, which ensures a particularly regular drawing-blowing operation and receptacles of very high quality.